The present invention relates to the field of electronic circuit components. More specifically, the invention relates to a method for calibrating individual potentiometers.
Potentiometers are often used to vary the electrical resistance at a particular point in an electric circuit over a range determined by the physical characteristics of the potentiometer itself. Typically, potentiometers are made from an electrically resistive material generally having a connection point at each end, thereby providing a fixed electrical resistance between the two ends. A "wiper" is associated with the potentiometer and is slidably movable between the two ends of the resistive material. The wiper includes a connection point that provides a selectively variable resistance range from approximately the minimum resistance of the potentiometer to the full resistance of the potentiometer. Generally, potentiometers are designated by the maximum possible electrical resistance of the potentiometer, such as 1K, 5K, 10K, 100K ohms, etc.
In addition to the maximum electrical resistance, potentiometers are further designated by tolerance ranges such as +/-20%, +/-10%, +/-5% or +/-1%. As is the ase with almost any electrical component, it is generally less expensive to produce a potentiometer having a +/-20% tolerance than to produce a potentiometer having a +/-5% tolerance. In this respect, there is often a large trade-off between the accuracy of the potentiometer and the cost of the potentiometer.
Additionally, it is also common for the linearity of the potentiometer resistance to vary over the length of the resistive material used in the potentiometer, such that equal movements of the wiper do not always produce equal changes in the resistance. Thus, each potentiometer, regardless of its manufacturing technique, stated tolerance, and quality control, has individual characteristics with respect to its maximum electrical resistance, linearity, and the length of travel of the wiper between its maximum and minimum resistance.
Potentiometers are often used to provide a variable electrical resistance that is critical to the proper operation of the electrical circuit within which it is installed. In these types of applications, the potentiometer is generally provided with a pointer or indicator that provides an indication of the relative position of the wiper with respect to the resistive material of the potentiometer. The indicator interfaces with printed indicia in the form of numbers, letters or graduation marks on a surface in fixed relationship with the potentiometer. The interaction between the indicia and the potentiometer indicator provides a relative indication of the electrical resistance at the wiper connection and thereby the resistance of the potentiometer. Typically, when a potentiometer is manufactured, the indicia are printed on the potentiometer without any testing or measurement of the actual resistive characteristics of the potentiometer.
When a potentiometer is connected in an electrical circuit, changing the potentiometer resistance can have a large effect on the output or function of the circuit in which it is installed. Therefore, in a situation where the electrical resistance of the potentiometer is critical to the performance of the circuit, the interface relationship between the indicator on the potentiometer and the printed indicia must accurately indicate the true electrical resistance of the potentiometer. In applications where relatively small changes in the resistance of the potentiometer have larger effects on the circuit itself, further calibration of the potentiometer is often required.
In this regard, the Rogers et al. U.S. Pat. No. 5,565,785 provides a method for calibrating a potentiometer that provides indicia reflecting a relatively accurate indication of the electrical resistance throughout the resistance range of the particular potentiometer. The '785 patent describes measuring the maximum resistance of the potentiometer, a first resistance at a predetermined location, and a second resistance at some known angle from the first resistance. Based on these three measured resistances and the known angle between two of the resistance values, a series of preset index numbers are used to determine the spacing between the indicia printed on the potentiometer face. In this manner, simple and quick electrical measurements of resistance values and one angle measurement can be used to calibrate the potentiometer.
Although the above-identified method of calibrating a potentiometer somewhat tailors the printed indicia according to the resistance value of the individual potentiometer, the limited number of measurements made on the potentiometer do not provide enough accuracy for many uses of the potentiometer. Therefore, it is particularly desirable to develop a method for calibrating a potentiometer that more accurately determines the potentiometer's resistance values at a larger number of angular positions of the wiper.
It is an object of the present invention to provide a method of calibrating a potentiometer that determines the electrical resistance of the potentiometer at a relatively large number of angular positions of the wiper. It is another object of the invention to provide a method of calibrating a potentiometer that compensates for the inherent hysteresis found in a potentiometer and the inherent hysteresis of the measuring device as a result of the wiper flexibility when the wiper is moved in opposite directions within the potentiometer. Finally, it is an object of the invention to provide a method of calibrating a potentiometer that can quickly and accurately produce an average set of resistance values for the entire range of wiper movement within the potentiometer itself.